liblp: Don't halve super partition for virtual a/b devices

For Virtual A/B devices, we snapshot the partitions. Hence
there is no need to prioritize using the second half of super
partition for _b partitions.

Test: liblp_test_static
Bug: 135752105

Change-Id: I8bd669e6ffa0a4d114a1b386133179bd2819099b